The prequel, which author George R.R. Martin said is called “The Long Night,” comes from Martin and showrunner Jane Goldman (“Kingsman: The Secret Service”) and is set 8,000 years before the events of “Game of Thrones.”

HBO has described the premise as “the world’s descent from the Golden Age of Heroes into its darkest hour . . . From the horrifying secrets of Westeros’ history to the true origin of the White Walkers, the mysteries of the East, to the Starks of legend — it’s not the story we think we know.”

In other words, don’t expect Jon Snow (Kit Harington) to pop up. But that doesn’t mean Watts will play a totally unknown character. Put on your nerd hats — here are the most likely possibilities.

1. A Lannister ancestor

Watts looks enough like Jaime and Cersei to play their mom. Of course, since the prequel takes place 8 millennia before Joanna Lannister lived, this isn’t possible, but Watts could be a distant forebear.

2. Daenerys Targaryen’s great-great-great grandmother

OK, there are probably about 10 more “greats” in there, but you get the point. The madness that runs in Targaryen blood might just be the “dark secret” in Watts’ character description.

3. The Night’s Queen

Remember the Night King, the creepy silent figure who was last seen killing (and resurrecting) Dany’s dragon at the end of Season 7? In the books’ lore, there’s a figure called the Night’s Queen, who lived exactly 8,000 earlier. Hmm.

4. Nissa Nissa

You might recall that in present day “Games of Thrones,” Melisandre (Carice van Houten) believes Jon Snow is a reborn version of legendary hero Azor Ahai. The prequel is set during Ahai’s era, and if he appears in the new series, he seems likely to carry out his tragic love story with a woman named Nissa Nissa.

5. Eleni

Eleni is a figure in the story’s lore who’s supposedly the daughter of the sea god and the goddess of the wind. She lived during the Age of Heroes, so the timeline checks out. She also committed herself to a mortal man, displeasing her parents. That would certainly be a “dark secret.”

Of course, Watts might also turn out to be the show’s Ned Stark (Sean Bean) — seemingly important, only to be killed off in the first season.